In recent years, for the purpose of achieving both of collision safety and reduction in fuel consumption by vehicle weight saving, GA steel sheets used for automobile steel sheets have been required to have a high strength of 1180 MPa or more and have also been required to have excellent formability (particularly bendability) represented by press forming.
However, improvement in strength is liable to cause deterioration of formability (particularly bendability). Accordingly, when used for the automobile steel sheets requiring complicated working, GA steel sheets have been required, which are relatively easily formed during forming and have a large amount of bake hardening in coating and baking after the forming to achieve high strength, as a property.
The bake hardening is a phenomenon that carbon or nitrogen dissolved in steel is fixed to a dislocation during coating by baking to increase yield strength, and it is known that the higher amount of solute carbon or nitrogen and the higher dislocation density cause the increase in bake hardenability (for example, see Patent Document 1). Martensite or bainite which is often used in order to realize the high strength is high in dislocation density compared to ferrite, and is high in the bake hardenability immediately after production, because carbon is supersaturatedly dissolved in solid.
On the other hand, the GA steel sheet is cooled to a temperature range of 100 to 600° C. after annealing at high temperature, and immersed in a galvanizing bath. Usually, before immersion in the galvanizing bath or during immersion in the galvanizing bath, a transformed structure of martensite or bainite is formed from austenite, and a carbide or a nitride is precipitated by a subsequent alloying treatment at about 500° C. to decrease the amount of solute carbon or nitrogen and to decrease the dislocation density by recovery. For this reason, in the GA steel sheet having martensite or bainite as a main phase, it has been difficult to realize the high bake hardenability.
Further, although the high strength can be realized in the steel sheet having martensite or bainite as a main phase, it has been difficult to achieve both of the bake hardenability and bendability. That is, martensite has a large amount of solute carbon and also has the high dislocation density compared to bainite, so that it is superior in the bake hardenability, but poor in ductility and inferior in the bendability. On the other hand, when bainite is excessively contained, the bendability is improved, but there is a problem that the bake hardenability is deteriorated.
For this reason, in the high-strength GA steel sheet having a tensile strength of 1180 MPa or more, a technique for achieving both of the bake hardenability and the bendability has not been established.
For example, Patent Document 1 discloses a GA steel sheet containing ferrite as a main phase and a hard phase (martensite or bainite) finely dispersed therein. In this GA steel sheet, it is considered that C in a hard structure is dispersed in the ferrite by performing additional heat treatment at 300 to 450° C. after annealing of the steel sheet to increase the amount of solute C in ferrite, thereby enhancing the bake hardenability. However, this GA steel sheet has ferrite as the main phase, so that the tensile strength thereof is as low as about 500 to 600 MPa, and a tensile strength of 1180 MPa or more, which is targeted in the present invention, cannot be achieved. In the Examples of the same document, a GA steel sheet of a martensite single phase (steel No. Q-3 in Table 6) is disclosed as a Comparative Example. In this GA steel sheet, the Mn content is as high as 3.2 mass % (steel No. Q in Table 1). From this, it is assumed to be a martensite single phase steel in which austenite is stabilized and which is in a substantially quenched state, to have the high dislocation density and a large amount of solute carbon, and to have the high bake hardenability. However, this GA steel sheet contains no bainite, and it is not considered that the sufficient bendability is obtained.
Furthermore, Patent Document 2 discloses a technique of obtaining a structure composed of martensite and low-temperature produced bainite by heat treatment of completely austenitizing a steel material by heating at the Ac3 point or higher and subsequent cooling to the Ms point or lower, thereby increasing the dislocation density. However, from the Examples of the same document, it is apparent that this is substantially a heat treatment technique for a cold-rolled sheet, and this is a technique which cannot be realized for the GA steel sheet.
In addition, Patent Document 3 discloses a high-strength cold-rolled steel sheet in which high yield strength and high elongation are considered to be realized by adjusting the dislocation density in the whole structure to 1×1015 to 1×1016 m−2. However, the structure of this steel sheet is composed of martensite and ferrite tempered at a temperature of higher than 400° C. It is therefore not conceivable that a sufficient amount of solute carbon is ensured, and the excellent bake hardenability cannot be developed.
Moreover, Patent Document 4 discloses a high-strength cold-rolled steel sheet in which both of the high strength and the high bendability are considered to be realized by obtaining a low-temperature tempered martensite single phase steel. However, in this steel sheet, tempering of 120 seconds or more at 100° C. or higher is essential. It is therefore not conceivable that sufficient solute carbon is ensured, and the excellent bake hardenability cannot be developed.